merge-ort: split out a separate display_update_messages() function

merge-ort: split out a separate display_update_messages() function

This patch includes no new code; it simply moves a bunch of lines into a
new function.  As such, there are no functional changes.  This is just a
preparatory step to allow the printed messages to be handled differently
by other callers, such as in `git merge-tree --write-tree`.

(Patch best viewed with
     --color-moved --color-moved-ws=allow-indentation-change
 to see that it is a simple code movement.)

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>